Prior to processing a silicon wafer to make semiconductor devices, the wafer is inscribed with a bar code. As the wafer is processed, the bar code is read to track the wafer to determine what process steps have been performed. As compared to the conventional bar code printed on consumer products, such as grocery items, the bar code inscribed on a wafer is typically smaller in order to maximize the usable area on the wafer. Moreover, the contrast of the bar code on the wafer is reduced as compared to the bar code printed on a grocery item for example.
The reduced size and contrast of the bar code on a semiconductor wafer has necessitated the development of specialized bar code readers. One example of a bar code reader designed specifically to read semiconductor wafers is disclosed in U.S. Pat. No. 4,585,931, issued on Apr. 29, 1986, in the names of H. A. Duncan et al. and assigned to AT&T Technologies, Inc. The bar code reader disclosed in the Duncan et al. patent includes a rotating turntable for supporting the wafer. A source of radiant light energy serves to direct a beam of radiant light energy at a predetermined angle to the wafer surface so as to impinge on different portions of the area on the wafer containing the bar code as the wafer is rotated. A light sensor, in the form of a photocell, senses the light energy reflected from the wafer. A bar code decoder interprets the output signal of the photocell to decode the bar code on the wafer.
While the Duncan et al. bar code reader is effective to read the bar code on a semiconductor wafer, the wafer must be positioned in relatively close proximity to the light source and light sensor if the Duncan et al. reader is to operate properly. Unfortunately, during certain processing operations, the wafer cannot be situated in proximity to a conventional bar code reader, such as the one disclosed in the Duncan et al. patent. Thus, during such operations, it is not possible to detect the bar code on the wafer using such present-day readers. Moreover, the Duncan et al. bar code requires that a mechanism be provided to rotate the wafer, which adds to the reader cost and makes incorporation of the reader in various semiconductor processing applications more difficult.
Thus them is a need for a technique for reading a bar code on a remotely situated object, such as a semiconductor wafer.